Coating material for secondary battery separator and manufacturing method thereof, secondary battery separator, and secondary battery

Abstract

Provided is a coating material for a secondary battery separator, the coating material including surface-treated spherical silica particles that have on the surfaces thereof a R1SiO3 / 2 unit (in the formula, R1 is a univalent hydrocarbon group having 1–20 substituted or unsubstituted carbon atoms) and a R2 3SiO1 / 2 unit (in the formula, R2 is a univalent hydrocarbon group having 1–16 substituted or unsubstituted carbon atoms that are mutually the same or different), have a median diameter of 0.01–0.5 μm in a volume-based particle size distribution, and have a circularity of 0.8–1.0.

Description

[Technical Field]

[0001] The present invention relates to a coating material for a secondary battery separator and a method for producing the same, a secondary battery separator, and a secondary battery. [Background technology]

[0002] In recent years, with the trend toward cordless electronic devices, the development of high-performance secondary batteries has been actively promoted, and secondary batteries that can be used repeatedly by recharging are used in a variety of devices. Examples of secondary batteries include nickel-cadmium batteries, nickel-metal hydride batteries, and lithium-ion batteries.

[0003] A secondary battery has two electrodes, an anode and a cathode, immersed in an electrolyte solution, and a separator separating them. Each type of secondary battery is optimized based on the electrode material, electrolyte, and separator used. The separator, in particular, must not only structurally separate the anode and cathode, but also electrically insulate them to prevent internal short circuits. The separator must also be permeable to allow ions to pass between the anode and cathode to carry out the electrochemical reaction.

[0004] In recent years, efforts to improve the energy density of batteries have been active, and separators must have as low an internal resistance as possible to maximize their ion permeability. While separators are required to have such high ion permeability, they are also required to be as thin as possible for the devices in which they are used. In consideration of the capabilities required of such separators, separators based on nonwoven fabrics have been developed.

[0005] Nonwoven fabric separators are more heat-resistant than commonly used polyolefin-based separators, and their high porosity allows for high current density. These advantages are extremely useful for secondary batteries. However, such high porosity can lead to micro-short circuits or short circuits under long-term repeated charge-discharge cycles.

[0006] For these reasons, nonwoven fabric separators are required to have the ability to prevent short circuits without impairing high ion permeability. One technique that can satisfy both requirements is to appropriately control the porosity, but for nonwoven fabrics with various pore sizes, the porosity must be appropriately controlled according to the pore size.

[0007] Preventing combustion is also important in lithium-ion batteries. Thin, laminated lithium-ion batteries, in particular, have adjacent battery units, which can ignite due to short circuits or overheating, potentially spreading the fire. To prevent battery fires caused by overheating or damage, the anode and cathode must remain electrically insulated from each other, even in the event of a fire. Therefore, separators must be able to maintain their insulation and survive even in the unlikely event of a fire.

[0008] Patent Document 1 proposes a heat-resistant separator having a structure in which various fillers are applied to a porous polyolefin resin film. However, although the document describes the heat resistance of the film, it does not mention its resistance to combustion. Furthermore, Patent Document 2 proposes a film in which inorganic spherical particles such as titanium oxide are coated on a polypropylene film. It is described that when this film is used as a separator, coating the particles can prevent internal short circuits, but there is no description regarding combustion or heat resistance. In either case, there is a difference in current density compared to nonwoven fabrics, and it is difficult to prevent short circuits even under high current densities and to maintain insulation even if combustion occurs. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-173283 [Patent Document 2] Special Publication No. 2018-538164 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention has been made in view of the above circumstances, and aims to provide a coating material for a secondary battery separator that provides a secondary battery that does not impair battery characteristics during long-term charge / discharge cycles and can maintain insulation even if combustion occurs. [Means for solving the problem]

[0011] As a result of intensive research to achieve the above object, the inventors have discovered that by applying a coating material containing specific surface-treated spherical silica particles to a separator, a secondary battery can be realized that does not impair battery characteristics even during long-term charge-discharge cycles and can maintain insulation even if combustion occurs, and have completed the present invention.

[0012] That is, the present invention is 1. R on the surface 1 SiO 3 / 2 Units (wherein R 1 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 20 carbon atoms, and R 2 3SiO 1 / 2 Units (wherein R 2 are the same or different, substituted or unsubstituted monovalent hydrocarbon groups having 1 to 6 carbon atoms, and the surface-treated spherical silica particles have a median diameter of 0.01 to 0.5 μm in a volume-based particle size distribution and a circularity of 0.8 to 1.0; 2. A method for producing a coating material for a secondary battery separator according to claim 1, The following formula (I) Si(OR 3 )4(I) (In the formula, R 3 are the same or different monovalent hydrocarbon groups having 1 to 6 carbon atoms.) a tetrafunctional silane compound represented by the formula (II) below, a partial hydrolysis product thereof, or a mixture thereof is hydrolyzed and condensed in a mixed solution of a hydrophilic organic solvent and water in the presence of a basic substance to obtain a mixed solvent dispersion of hydrophilic spherical silica particles containing SiO2 units, and then the mixed solvent dispersion of the hydrophilic spherical silica particles is added with a compound represented by the formula (II) below: R 1 Si(OR 4 )3(II) (In the formula, R 1 is the same as above, and R 4 are the same or different monovalent hydrocarbon groups having 1 to 6 carbon atoms.) The surface of the hydrophilic spherical silica particles is treated with a trifunctional silane compound represented by the formula: 1 SiO 3 / 2 Units (wherein R 1 is the same as above.) is introduced into the first surface-treated spherical silica particles in a mixed solvent, and then the first surface-treated spherical silica particles are concentrated by removing a part of the hydrophilic organic solvent and water from the mixed solvent dispersion to obtain a concentrated mixed solvent dispersion of the first surface-treated spherical silica particles. Then, the first surface-treated spherical silica particles are concentrated in a mixed solvent by adding a compound represented by the following formula (III) to the concentrated mixed solvent dispersion of the first surface-treated spherical silica particles. R 2 3SiNHSiR 2 3(III) (In the formula, R 2 is the same as above.) a silazane compound represented by the following formula (IV): R 2 3SiX (IV) (In the formula, R 2 is the same as above, and X is an OH group or a hydrolyzable group. or a mixture thereof, to treat the surface of the first surface-treated spherical silica particles, thereby forming a monofunctional silane compound represented by R 2 3SiO 1 / 2 Units (wherein R 2 is the same as above.) to obtain surface-treated spherical silica particles as second surface-treated spherical silica particles; 3. A substrate and a film formed on the surface of the substrate or inside the pores of the substrate. Ta1 A secondary battery separator having a coating of the coating material for secondary battery separators described above, 2 a secondary battery separator in which the amount of the surface-treated spherical silica particles per separator is 0.07 to 0.29 mg; 4. The substrate is nonwoven fabric. 3 The secondary battery separator according to claim 1, 5. A secondary battery comprising the secondary battery separator according to 3 or 4. to provide. [Effects of the Invention]

[0013] According to the present invention, by applying a coating material containing specific surface-treated spherical silica particles to a secondary battery separator, the battery characteristics over a long-term charge-discharge cycle can be improved, and in the event of a fire, the spread of fire in the separator can be prevented, and the anode and cathode can be electrically separated and maintained in insulation even after the fire, thereby providing a highly reliable secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in detail below. The coating material for a secondary battery separator according to the present invention has a surface with R 1 SiO 3 / 2 Units and R 2 3SiO 1 / 2 The particle size distribution includes surface-treated spherical silica particles having a predetermined median diameter in a predetermined volume-based particle size distribution and a predetermined circularity.

[0015] [Surface-treated spherical silica particles] The surface-treated spherical silica particles used in the present invention have excellent dispersibility, allowing for uniform coating on the separator substrate. Furthermore, the surface-treated spherical silica particles used in the present invention penetrate into the pores of the separator substrate, thereby preventing a significant increase in separator film thickness due to the accumulation of aggregated particles on the coating surface. Avoiding an increase in separator thickness is extremely important from the perspectives of internal electrical resistance and the requirements of the device in which the separator is used.

[0016] [1] Surface treatment agent The above R on the silica particle surface 1 SiO 3 / 2 Units and R 2 3SiO 1 / 2 The unit can be formed by bonding a trifunctional silane compound represented by the following formula (II), its partial hydrolysis product, or a mixture thereof, and a silazane compound represented by the following formula (III), a monofunctional silane compound represented by the following formula (IV), their hydrolysis product, their condensation product, or a mixture thereof, to the silica surface. R 1 Si(OR 4 )3(II) R 2 3SiNHSiR 2 3(III) R 2 3SiX (IV)

[0017] Above R 1 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 20 carbon atoms, preferably 1 to 6 carbon atoms, more preferably 1 to 3 carbon atoms, and even more preferably 1 or 2 carbon atoms. R 1Specific examples of the monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, and n-hexyl, among which methyl, ethyl, n-propyl, and isopropyl groups are preferred, and methyl and ethyl groups are particularly preferred. Some or all of the hydrogen atoms in these monovalent hydrocarbon groups may be substituted with halogen atoms such as fluorine, chlorine, and bromine atoms, preferably fluorine atoms.

[0018] Above R 2 are the same or different, substituted or unsubstituted monovalent hydrocarbon groups having 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms, and more preferably 1 or 2 carbon atoms. R 2 Specific examples of the monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, n-propyl, isopropyl, and n-butyl, among which methyl, ethyl, and propyl groups are preferred, and methyl and ethyl groups are particularly preferred. Some or all of the hydrogen atoms in these monovalent hydrocarbon groups may be substituted with halogen atoms such as fluorine, chlorine, and bromine atoms, preferably fluorine atoms.

[0019] In the above formula (II), R 4 are the same or different monovalent hydrocarbon groups having 1 to 6 carbon atoms, preferably 1 to 3 carbon atoms, and more preferably 1 or 2 carbon atoms. R 4 Specific examples of the monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, n-propyl, and n-butyl groups, and among these, methyl, ethyl, and propyl groups are preferred, and methyl and ethyl groups are particularly preferred.

[0020] In the above formula (IV), X is an OH group or a hydrolyzable group. Specific examples of the hydrolyzable group for X include halogen atoms such as chlorine atom and bromine atom, alkoxy groups such as methoxy and ethoxy groups, amino groups such as dimethylamino and diethylamino groups, and acyloxy groups such as acetoxy group. Among these, alkoxy groups and amino groups are preferred, and methoxy and ethoxy groups are particularly preferred.

[0021] Examples of the trifunctional silane compound represented by the above formula (II) include trialkoxysilanes such as methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, isopropyltrimethoxysilane, isopropyltriethoxysilane, n-butyltrimethoxysilane, n-butyltriethoxysilane, n-hexyltrimethoxysilane, trifluoropropyltrimethoxysilane, and heptadecafluorodecyltrimethoxysilane. These can be used alone or in combination of two or more. Among these, preferred are methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, and ethyltriethoxysilane, and more preferred are methyltrimethoxysilane, methyltriethoxysilane, or their partial hydrolysis (condensation) products.

[0022] Examples of the silazane compound represented by the formula (III) include hexamethyldisilazane and hexaethyldisilazane, which can be used alone or in combination of two or more. Among these, hexamethyldisilazane is preferred.

[0023] Examples of the monofunctional silane compound represented by the formula (IV) include monosilanol compounds such as trimethylsilanol and triethylsilanol; monochlorosilanes such as trimethylchlorosilane and triethylchlorosilane; monoalkoxysilanes such as trimethylmethoxysilane and trimethylethoxysilane; monoaminosilanes such as trimethylsilyldimethylamine and trimethylsilyldiethylamine; and monoacyloxysilanes such as trimethylacetoxysilane. These can be used alone or in combination of two or more. Among these, preferred are trimethylsilanol, trimethylmethoxysilane, and trimethylsilyldiethylamine, and particularly preferred are trimethylsilanol and trimethylmethoxysilane.

[0024] [2] Silica particles Synthetic silica particles are broadly classified into combustion silica, deflagration silica, wet silica, and sol-gel silica (the so-called Stoeber method) according to their production method. Among these, sol-gel silica is particularly suitable as a coating material for the secondary battery separator of the present invention because it has excellent monodispersity, a spherical shape, and a widely controllable particle size. Furthermore, the interior of the silica core is porous, so that ion mobility is not inhibited when immersed in an electrolyte.

[0025] [3] Method for manufacturing surface-treated silica particles An example of a preferred method for producing surface-treated spherical silica particles by the sol-gel method will be described below.

[0026] The surface-treated spherical silica particles used in the present invention may be, for example, Step (A1): synthesis of hydrophilic silica particles; Step (A2): Surface treatment step using a trifunctional silane compound; Step (A3): Concentration step, Step (A4): Surface treatment step with a monofunctional silane compound It can be obtained by a production method including the steps of:

[0027] That is, the method for producing the surface-treated spherical silica particles used in the present invention is as follows: Step (A1): A compound represented by the following formula (I) Si(OR 3 )4(I) (In the formula, R 3 are the same or different monovalent hydrocarbon groups having 1 to 6 carbon atoms.) a step of hydrolyzing and condensing a tetrafunctional silane compound represented by the formula: Step (A2): Adding a compound represented by the following formula (II) to the mixed solvent dispersion of the hydrophilic silica particles: R 1 Si(OR 4 )3(II) (In the formula, R 1 and R 4 is the same as above.) The surface of the hydrophilic silica particles is treated with a trifunctional silane compound represented by the formula: 1 SiO 3 / 2 Units (wherein R 1 is the same as above.) to obtain a dispersion of the first surface-treated silica particles in the mixed solvent; step (A3): removing a portion of the hydrophilic organic solvent and water from the first dispersion of surface-treated silica particles in the mixed solvent and concentrating the mixture to obtain a concentrated dispersion of surface-treated silica particles in the mixed solvent; Step (A4): A silazane compound represented by the following formula (III), a monofunctional silane compound represented by the following formula (IV), or a mixture thereof is added to the concentrated dispersion of the first surface-treated silica particles in a mixed solvent. R 2 3SiNHSiR 2 3(III) R 2 3SiX (IV) (In the formula, R 2 and X are the same as above.) and further treating the surfaces of the first surface-treated silica particles by adding R 2 3SiO 1 / 2 Units (wherein R 2 is the same as above.) to obtain second surface-treated silica particles. Hereinafter, each of the steps (A1) to (A4) will be explained in order.

[0028] [3-1] Step (A1): Synthesis of hydrophilic silica particles In this step, a compound of formula (I): Si(OR 3 )4(I) A tetrafunctional silane compound represented by the formula (I), its partial hydrolysis (condensation) product, or a mixture of these is hydrolyzed and condensed in a mixture of a hydrophilic organic solvent and water in the presence of a basic substance to obtain a hydrophilic silica particle mixed solvent dispersion.

[0029] In formula (I), R 3 are the same or different monovalent hydrocarbon groups having 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms, and more preferably 1 or 2 carbon atoms. R 3 Specific examples of the monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, n-propyl, and n-butyl groups; and aryl groups such as phenyl groups. Among these, methyl, ethyl, n-propyl, and n-butyl groups are preferred, and methyl and ethyl groups are more preferred.

[0030] Examples of the tetrafunctional silane compound represented by the above formula (I) include tetraalkoxysilanes such as tetramethoxysilane, tetraethoxysilane, tetra-n-propoxysilane, and tetra-n-butoxysilane; and tetraaryloxysilanes such as tetraphenoxysilane, and these can be used alone or in combination of two or more. Among these, preferred are tetramethoxysilane, tetraethoxysilane, tetra-n-propoxysilane, and tetra-n-butoxysilane, and more preferred are tetramethoxysilane and tetraethoxysilane. Furthermore, examples of the partial hydrolysis condensation products of the tetrafunctional silane compound represented by formula (I) include methyl silicate, ethyl silicate, and the like.

[0031] The hydrophilic organic solvent is not particularly limited as long as it can dissolve the tetrafunctional silane compound represented by the above formula (I), its partial hydrolysis condensation product, and water, and for example, can include alcohols; cellosolves such as methyl cellosolve, ethyl cellosolve, butyl cellosolve, and cellosolve acetate; ketones such as acetone and methyl ethyl ketone; ethers such as dioxane and tetrahydrofuran, and can be used alone or in combination of two or more.Among these, alcohols and cellosolves are preferred, and alcohols are more preferred.

[0032] The alcohols include those represented by the following formula (V): R 5 OH (V) Examples of alcohols include those represented by the following formula:

[0033] In the above formula (V), R 5 is a monovalent hydrocarbon group having 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms, and more preferably 1 or 2 carbon atoms. R 5 Specific examples of the monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, n-propyl, isopropyl, and n-butyl groups, and among these, methyl, ethyl, n-propyl, and isopropyl groups are preferred, and methyl and ethyl groups are more preferred.

[0034] Examples of the alcohol represented by the above formula (V) include methanol, ethanol, propanol, isopropanol, butanol, etc., and among these, methanol and ethanol are preferred. As the number of carbon atoms in the alcohol increases, the particle size of the resulting silica particles tends to increase, so methanol is preferred for obtaining the desired small silica particles.

[0035] Examples of the basic substance include ammonia, dimethylamine, diethylamine, etc., and among these, ammonia and diethylamine are preferred, and ammonia is more preferred. A required amount of these basic substances may be dissolved in water, and the resulting aqueous solution (basic) may then be mixed with the hydrophilic organic solvent.

[0036] The amount of water used in this case is preferably 0.5 to 5 mol, more preferably 0.6 to 2 mol, and even more preferably 0.7 to 1 mol per mol of the total hydrocarbyloxy groups of the tetrafunctional silane compound represented by the above formula (I), its partial hydrolysis condensation product, or a mixture thereof. The molar ratio of the hydrophilic organic solvent to water is preferably 0.5 to 10, more preferably 3 to 9, and even more preferably 5 to 8. The larger the amount of the hydrophilic organic solvent, the smaller the desired silica particles will be.

[0037] The amount of the basic substance is preferably 0.01 to 2 mol, more preferably 0.02 to 0.5 mol, and even more preferably 0.04 to 0.12 mol, per mol of the total hydrocarbyloxy groups of the tetrafunctional silane compound represented by formula (I), its partial hydrolysis condensation product, or a mixture thereof. The smaller the amount of basic substance, the more likely it is that the desired small silica particles will be obtained, while a larger amount may result in large silica particles.

[0038] The hydrolysis and condensation of the tetrafunctional silane compound represented by formula (I) can be carried out by a known method, i.e., by adding the tetrafunctional silane compound represented by formula (I) to a mixture of water and a hydrophilic organic solvent containing a basic substance. The reaction can be carried out under known conditions, and is usually carried out at about 10 to 80°C for about 1 to 20 hours.

[0039] The concentration of silica particles in the hydrophilic silica particle mixed solvent dispersion obtained in this step (A1) is generally 3 to 15% by mass, and preferably 5 to 10% by mass.

[0040] [3-2] Step (A2): Surface treatment step using a trifunctional silane compound In this step, the hydrophilic silica particle mixed solvent dispersion obtained in step (A1) is added with a compound represented by the following formula (II): R 1 Si(OR 4 )3(II) The surface of the hydrophilic silica particles is treated with a trifunctional silane compound represented by the formula: 1 SiO 3 / 2 Units (wherein R 1 is the same as above.) is introduced to obtain a dispersion of the first surface-treated silica particles in the mixed solvent.

[0041] This step (A2) is essential for suppressing the aggregation of silica particles in the next step, the concentration step (A3). If this aggregation cannot be suppressed, the individual particles of the resulting silica-based powder will not be able to maintain their primary particle size, which may result in aggregation or uneven distribution of the silica particles when applied to a separator.

[0042] R in the above formula (II) 1 , R 4 Specific examples of the trifunctional silane compound represented by formula (II) are the same as those explained for the surface treatment agent for silica fine particles.

[0043] The amount of the trifunctional silane compound represented by the formula (II) added is preferably 0.001 to 1 mole, more preferably 0.01 to 0.1 mole, and even more preferably 0.01 to 0.05 mole per mole of Si atoms in the hydrophilic silica particles. When the amount added is 0.001 mole or more, dispersibility is improved. Furthermore, when the amount added is 1 mole or less, aggregation of the silica particles does not occur. The reaction conditions are preferably about 10 to 80°C and about 1 to 20 hours.

[0044] The concentration of the first surface-treated silica particles in the mixed solvent dispersion obtained in step (A2) is usually 3% by mass or more and less than 15% by mass, preferably 5 to 10% by mass. If the concentration is 3% by mass or more, productivity improves, and if it is less than 15% by mass, aggregation of the silica particles does not occur.

[0045] [3-3] Process (A3) Concentration process In this step, the hydrophilic organic solvent and a portion of the water are removed from the first surface-treated silica particle mixed solvent dispersion obtained in step (A2), followed by concentration to obtain a first concentrated mixed solvent dispersion of surface-treated silica particles having a desired concentration. In this step, a hydrophobic organic solvent may be added beforehand or during the step. The hydrophobic solvent is preferably a hydrocarbon solvent, a ketone solvent, or the like. Specific examples include toluene, xylene, methyl ethyl ketone, and methyl isobutyl ketone, and these can be used alone or in combination of two or more. Among these, methyl isobutyl ketone is preferred. Examples of methods for removing a portion of the hydrophilic organic solvent and water include distillation and distillation under reduced pressure. The preferred conditions are a temperature of about 10 to 150°C for about 1 to 20 hours.

[0046] The resulting concentrated dispersion preferably has a silica particle concentration of 15 to 40% by mass, more preferably 20 to 35% by mass, and even more preferably 25 to 30% by mass. If the silica particle concentration is 15% by mass or more, the subsequent surface treatment can be carried out smoothly, and if it is 40% by mass or less, aggregation of the silica particles does not occur.

[0047] This step (A3) is essential for preventing problems such as the silazane compound represented by formula (III) and the monofunctional silane compound represented by formula (IV), which are used as surface treatment agents in the next step (A4), reacting with alcohol or water to result in insufficient surface treatment, causing aggregation during subsequent drying, and preventing the resulting silica powder from maintaining its primary particle size.

[0048] [3-4] Step (A4): Surface treatment step using a monofunctional silane compound In this step, a silazane compound represented by the following formula (III), a monofunctional silane compound represented by the following formula (IV), or a mixture thereof is added to the concentrated dispersion of the first surface-treated silica particles in the mixed solvent obtained in step (A3), and the surfaces of the first surface-treated silica particles are further surface-treated to form R 2 3SiO 1 / 2 Units (wherein R 2 is the same as above.) is introduced to obtain the second surface-treated silica particles. R 2 3SiNHSiR 2 3(III) R 2 3SiX (IV) In this step, the silanol groups remaining on the surface of the first surface-treated silica particles are triorganosilylated to form R 2 3SiO 1 / 2 The units are introduced to the surface.

[0049] R in the above formula 2 Specific examples of X, the silazane compound represented by formula (III) and the monofunctional silane compound represented by formula (IV) are the same as those explained for the surface treatment agent for silica fine particles.

[0050] The amount of the silazane compound or monofunctional silane compound used is preferably 0.1 to 0.5 mol, more preferably 0.2 to 0.4 mol, and particularly preferably 0.25 to 0.35 mol, per mol of Si atoms in the hydrophilic silica particles. When the amount used is 0.1 mol or more, good dispersibility is achieved. Furthermore, when the amount used is 0.5 mol or less, it is economically advantageous. The reaction conditions are preferably about 10 to 150°C for about 1 to 20 hours.

[0051] The surface-treated spherical silica particles obtained can be used as a coating material for secondary battery separators as they are, but it is preferable to use a dispersion prepared by mixing an appropriate amount of the surface-treated spherical silica particles with a solvent. The surface-treated spherical silica particles have excellent dispersibility and maintain a high dispersion state even when mixed in a solvent, which not only facilitates application to separator substrates but also enables uniform application. Before the mixing step, it is preferable to dehydrate the surface-treated spherical silica particles, preferably by heating. Specifically, it is preferable to provide a dehydration step in which the particles are dried at a temperature of 160 to 260°C under normal or reduced pressure for 12 hours or more.

[0052] The surface-treated spherical silica particles used in the present invention have a median diameter (50% cumulative diameter) in the volumetric particle size distribution of 0.01 to 0.5 μm, preferably 0.01 to 0.4 μm, and more preferably 0.01 to 0.35 μm. The method for measuring the median diameter in the volumetric particle size distribution will be described later.

[0053] The surface-treated silica particles used in the present invention have a spherical shape. In the present invention, "spherical" means that the circularity is 0.8 to 1.0. The circularity is preferably 0.8 to 0.95. The method for measuring the circularity will be described later.

[0054] [Coating material for secondary battery separators] [1] Surface-treated spherical silica particles The coating material for secondary battery separators of the present invention contains the above-mentioned surface-treated spherical silica particles. The content of the surface-treated spherical silica particles is preferably 1 to 50 mass %, more preferably 2 to 40 mass %, based on the total amount of the coating material.

[0055] [2] Solvent In the coating material for a secondary battery separator of the present invention, it is preferable to use a solvent to disperse the surface-treated spherical silica particles in the coating material. As the solvent, alcohols, esters, carbonates, ketones, lactones, ethers, sulfoxides, amides, etc. can be used. Examples of alcohols include methanol, ethanol, and isopropanol. Examples of esters include ethyl acetate, methyl propionate, and butyl acetate. Examples of carbonates include propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate. Examples of ketones include methyl ethyl ketone, methyl isobutyl ketone, methyl propyl ketone, and diethyl ketone. Examples of lactones include γ-butyl lactone. Examples of ethers include trimethoxymethane, 1,2-dimethoxyethane, diethyl ether, 2-ethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of sulfoxides include dimethyl sulfoxide. Examples of amides include N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide, and N,N-dimethylacetamide. These can be used alone or in combination of two or more. Among these, amides such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide, and N,N-dimethylacetamide are preferred. When a solvent is used, the content thereof is preferably 10 to 80 mass %, more preferably 20 to 70 mass %, based on the total amount of the coating material.

[0056] [3] Binder A binder may be added to the coating material for a secondary battery separator of the present invention in order to bind the surface-treated spherical silica particles to the separator substrate. Examples of binders that can be used include, but are not limited to, vinylidene fluoride copolymer resins such as copolymers of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), copolymers of vinylidene fluoride (VDF), tetrafluoroethylene (TFE), and hexafluoropropylene (HFP), and copolymers of vinylidene fluoride (VDF), hexafluoropropylene (HFP), perfluoromethyl vinyl ether (PFMV), and tetrafluoroethylene (TFE); fluorine-based resins such as polytetrafluoroethylene (PTFE); fluororubbers; polymers such as styrene-butadiene rubber (SBR) and ethylene-propylene rubber (EPDM); styrene-acrylonitrile copolymers; polysaccharides such as carboxymethyl cellulose (CMC); polyimide resins; and silicone resins. These can be used alone or in combination of two or more. Among these, vinylidene fluoride copolymer resins are preferred. When a binder is used, the content thereof is preferably 1 to 20% by mass, and more preferably 2 to 10% by mass.

[0057] [4] Manufacturing method of coating material The coating material for secondary battery separators of the present invention can be obtained by mixing the surface-treated silica particles produced by a method including the above steps (A1) to (A4) with a solvent, a binder, etc., as needed, in accordance with a conventional method, and then subjecting the mixture to a degassing treatment, etc.

[0058] [Separator] The method for applying the coating material for a secondary battery separator of the present invention to a separator substrate is not particularly limited, but examples thereof include a bar coater method, a spin coating method, a dip coating method, an offset printing method, and a screen printing method.

[0059] The coating material for a secondary battery separator of the present invention can be applied to or impregnated into one or both surfaces of a separator substrate, followed by drying to remove excess solvent, thereby forming a coating of the coating material on the surface of the substrate and, if the substrate has pores, inside the pores, thereby obtaining a separator in which the surface-treated spherical silica particles are bound to the surface of the separator substrate or inside the pores.

[0060] The substrate of the separator is not particularly limited as long as it is one that is normally used in secondary batteries, but among them, nonwoven fabrics are preferably used. Examples of nonwoven fabrics include those with fiber diameters of 0.1 to 5 μm (e.g., 0.1 μm, 1 μm, 5 μm, etc.), which vary depending on the manufacturing method, but are not particularly limited in the present invention. In addition, examples of fibers include cellulose fibers, pulp fibers, carbon fibers, glass fibers, ceramic fibers, aramid fibers, vinylon fibers, and polyamide fibers. From the viewpoint of flame resistance, aramid fibers are preferred, as they have a higher fire spread prevention effect. The thickness of the substrate is preferably 30 μm or less, more preferably 20 μm or less. There is no particular lower limit, but the thickness is preferably 1 μm or more.

[0061] It is preferable to dehydrate the nonwoven fabric before applying the coating material for secondary battery separators of the present invention. Specifically, it is preferable to provide a dehydration treatment step in which the nonwoven fabric is dried at a temperature of 130 to 260°C, particularly 140 to 200°C, under normal or reduced pressure for 12 hours or more.

[0062] The amount of the surface-treated spherical silica particles applied to the separator substrate is 0.07 to 0.29 mg / cm 2 and 0.11 to 0.29 mg / cm 2 0.07 mg / cm is preferred. 2 If the density is less than 0.29 mg / cm, the resulting secondary battery separator will be inferior in battery characteristics and flame retardancy in long-term charge / discharge cycles. 2 If the thickness exceeds this value, the separator membrane thickness increases, causing a problem of increased internal electrical resistance. The thickness of the resulting separator is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 24 μm or less. The lower limit is not particularly limited, but is preferably 1 μm or more.

[0063] [Secondary battery] The secondary battery using the separator coated with the coating material of the present invention is not particularly limited, but preferably includes a positive electrode and a negative electrode, a separator inserted between these electrodes, and a non-aqueous electrolyte, and a lithium ion secondary battery is more preferable.

[0064] [1] Positive electrode The positive electrode material includes, as a positive electrode material, a positive electrode active material, a conductive agent, a binder, a viscosity modifier, and the like. The positive electrode active material may be lithium or a compound containing lithium, and can be used alone or in appropriate combination of two or more. Specific examples of the compound containing lithium include, for example, lithium composite oxides containing lithium. Among them, in order to increase the energy density, a lithium composite oxide mainly composed of Li p MetO2 is preferable. Here, Met is preferably at least one of cobalt, nickel, iron, and manganese, and p is usually a value within the range of 0.05 ≦ p ≦ 1.10. Specific examples of such lithium composite oxides include LiCoO2, LiNiO2, LiFeO2, Li q Ni r Co 1-r O2 (where the values of q and r vary depending on the charge and discharge state of the battery, and are usually 0 < q < 1, 0.7 < r ≦ 1).), LiNi 0.8 Co 0.1 Mn 0.1 O2, spinel-structured LiMn2O4, orthorhombic LiMnO2, and the like. Further, as a high voltage compatible type, LiMet s Mn 1-s O4 (0 < s < 1) is also used, and in this case, Met includes titanium, chromium, iron, cobalt, nickel, copper, zinc, and the like.

[0065] The lithium composite oxide can be prepared, for example, by pulverizing and mixing a carbonate, nitrate, oxide, or hydroxide of lithium and a carbonate, nitrate, oxide, or hydroxide of a transition metal according to the desired composition, and firing the mixture in an oxygen atmosphere at a temperature in the range of 600 to 1,000°C.

[0066] [2] Negative electrode The negative electrode material contains, as negative electrode materials, a negative electrode active material, a conductive agent, a binder, a viscosity modifier, etc. The negative electrode active material may be used alone or in combination of two or more types. Specific examples of the negative electrode active material include carbon materials such as non-graphitizable carbon, graphitizable carbon, graphite, pyrolytic carbons, cokes, glassy carbons, baked organic polymer compounds, carbon fiber, activated carbon, etc. Also included are materials capable of absorbing and releasing lithium and containing one or more constituent elements selected from metal elements and metalloid elements.

[0067] Examples of conductive agents that can be used include metal powders and fibers of Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, Si, etc., as well as graphite such as natural graphite, artificial graphite, various coke powders, mesophase carbon, vapor-grown carbon fiber, pitch-based carbon fiber, PAN-based carbon fiber, and various types of baked resins. These can be used alone or in combination of two or more types.

[0068] Examples of binders include polyimide resin, polyamide resin, polyamideimide resin, polyvinylidene fluoride (PVDF) resin, styrene-butadiene rubber (SBR), etc. These can be used alone or in combination of two or more.

[0069] Examples of viscosity modifiers include carboxymethyl cellulose, sodium polyacrylate, other acrylic polymers, fatty acid esters, etc. These may be used alone or in combination of two or more.

[0070] The preferred contents (mass % solid content) of each component in the positive electrode material are 90 to 98 mass % of the positive electrode active material, 0.5 to 5.0 mass % of the conductive agent, 0.5 to 5.0 mass % of the binder, and 0 to 3.0 mass % of the viscosity modifier. The preferred contents (mass % solid content) of each component in the negative electrode material are 75 to 98 mass % for the negative electrode active material, 1 to 20 mass % for the conductive agent, 1 to 20 mass % for the binder, and 0 to 3.0 mass % for the viscosity modifier.

[0071] [3] Nonaqueous electrolyte Examples of non-aqueous electrolytes include light metal salts, such as alkali metal salts (lithium salts, sodium salts, potassium salts, etc.), alkaline earth metal salts (magnesium salts, calcium salts, etc.), and aluminum salts, and one or more of these may be selected depending on the purpose. For example, specific examples of lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, CF3SO3Li, (CF3SO2)2NLi, C4F9SO3Li, CF3CO2Li, (CF3CO2)2NLi, C6F5SO3Li, and CF 17 Examples include SO3Li, (C2F5SO2)2NLi, (C4F9SO2)(CF3SO2)NLi, (FSO2C6F4)(CF3SO2)NLi, ((CF3)2CHOSO2)2NLi, (CF3SO2)3CLi, (3,5-(CF3)2C6F3)4BLi, LiCF3, LiAlCl4, and C4BO8Li, and one of these can be used alone or two or more can be used in combination.

[0072] When using a non-aqueous electrolyte containing a lithium salt, the non-aqueous solvent of the electrolyte is not particularly limited as long as it can be used as a non-aqueous electrolyte. Generally, aprotic high-dielectric constant solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, etc.; dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, dipropyl carbonate, diethyl ether, tetrahydrofuran, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,3-dioxolane, sulfolane, methyl sulfolane, acetonitrile, propionitrile, anisole, acetate esters such as methyl acetate, propionate esters, etc., aprotic low-viscosity solvents, etc. It is desirable to use these aprotic high-dielectric constant solvents and aprotic low-viscosity solvents in combination at an appropriate mixing ratio. Additionally, ionic liquids using imidazolium, ammonium, and pyridinium type cations can be used. The counter anion is not particularly limited, but may be BF4 - , PF6 - , (CF3SO2)2N - The ionic liquid can be used by mixing with the non-aqueous electrolyte solvent described above.

[0073] When a solid electrolyte or gel electrolyte is used, it is possible to use glass-based inorganic solid electrolytes, polyether gel, silicone gel, silicone polyether gel, acrylic gel, silicone acrylic gel, acrylonitrile gel, poly(vinylidene fluoride), etc. as polymer materials. These may be polymerized in advance or may be polymerized after injection. These may be used alone or as a mixture of two or more.

[0074] Furthermore, various additives may be added to the non-aqueous electrolyte solution as needed. Examples include vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, 4-vinylethylene carbonate, etc., which are intended to improve cycle life; biphenyl, alkylbiphenyl, cyclohexylbenzene, t-butylbenzene, diphenyl ether, benzofuran, etc., which are intended to prevent overcharging; and various carbonate compounds such as carbon dioxide gas, various carboxylic acid anhydrides, and various nitrogen- and sulfur-containing compounds, which are intended to deoxidize or dehydrate. Furthermore, compounds in which some of these compounds are fluorinated are also suitable.

[0075] The nonaqueous electrolyte secondary battery includes a battery case that seals the above-mentioned battery configuration, and may have any shape without any particular limitations. Typical examples include a coin-type battery in which electrodes punched into a coin shape and a separator are laminated together, and a prismatic or cylindrical battery in which an electrode sheet and a separator are spirally wound. [Example]

[0076] The present invention will be specifically explained below by showing synthesis examples, examples and comparative examples, but the following examples do not limit the present invention in any way.

[0077] [1] Synthesis and evaluation of surface-treated silica particles [Synthesis Example 1] Step (S1): Synthesis of hydrophilic silica particles A 3-liter glass reactor equipped with a stirrer, dropping funnel, and thermometer was charged with 989.5 g of methanol, 135.5 g of water, and 66.5 g of 28% by mass aqueous ammonia, and mixed. The solution was adjusted to 35°C, and 436.5 g (2.87 mol) of tetramethoxysilane was added dropwise over 6 hours while stirring. After the dropwise addition was completed, stirring was continued for another 0.5 hours to allow hydrolysis to occur, yielding a suspension of hydrophilic silica particles.

[0078] Step (S2): Surface treatment step using a trifunctional silane compound To the suspension obtained in the above step (S1), 4.4 g (0.03 mol) of methyltrimethoxysilane was added dropwise at 25°C over 0.5 hours, and stirring was continued for 12 hours after the addition to treat the surfaces of the silica particles, thereby obtaining a first surface-treated silica particle dispersion.

[0079] ·Step (S3): Concentration step Next, an ester adapter and a condenser were attached to the glass reactor, and the dispersion obtained in the previous step was heated to 60-70°C to distill off 1,021 g of the methanol and water mixture, thereby obtaining a first concentrated dispersion of surface-treated silica particles in the mixed solvent. At this time, the silica particle content in the concentrated dispersion was 28 mass%.

[0080] Step (S4): Surface treatment step using a monofunctional silane compound To the concentrated dispersion obtained in the previous step, 138.4 g (0.86 mol) of hexamethyldisilazane was added at 25°C, and the dispersion was then heated to 50-60°C and reacted for 9 hours to trimethylsilylate the silica particles in the dispersion. The solvent in the dispersion was then distilled off at 130°C under reduced pressure (6,650 Pa) to obtain 186 g of second surface-treated silica particles [1].

[0081] [Synthesis Example 2] 188 g of surface-treated silica particles [2] were obtained in the same manner as in Synthesis Example 1, except that in step (S1), the amounts of methanol, water, and 28% by mass ammonia water were changed to 1,045.7 g of methanol, 112.6 g of water, and 33.2 g of 28% by mass ammonia water.

[0082] [Synthesis Example 3] Step (S1): Synthesis of hydrophilic silica particles A 3-liter glass reactor equipped with a stirrer, dropping funnel, and thermometer was charged with 623.7 g of methanol, 41.4 g of water, and 49.8 g of 28% by weight aqueous ammonia, and mixed. The solution was adjusted to 35°C, and while stirring, 1,163.7 g of tetramethoxysilane and 418.1 g of 5.4% by weight aqueous ammonia were simultaneously added dropwise, the former over 6 hours and the latter over 4 hours. Stirring was continued for 0.5 hours after the addition of tetramethoxysilane, allowing hydrolysis to occur, resulting in a suspension of hydrophilic silica particles.

[0083] Step (S2): Surface treatment step using a trifunctional silane compound To the suspension obtained in the above step (S1), 11.6 g of methyltrimethoxysilane (equivalent to a molar ratio of 0.01 relative to tetramethoxysilane) was added dropwise at 25°C over 0.5 hours, and the mixture was stirred for 12 hours after the addition to obtain first surface-treated silica particles.

[0084] ·Step (S3): Concentration step Next, an ester adapter and a condenser were attached to the glass reactor, and 1,440 g of methyl isobutyl ketone was added to the dispersion obtained in the previous step. The mixture was then heated to 80 to 110°C, and the mixture of methanol and water was distilled off over 7 hours, thereby obtaining a concentrated mixed solvent dispersion of first surface-treated silica particles.

[0085] Step (S4): Surface treatment step using a monofunctional silane compound To the concentrated dispersion obtained in the previous step, 357.6 g of hexamethyldisilazane was added at 25°C, and the mixture was heated to 120°C and reacted for 3 hours to trimethylsilylate the silica particles. Thereafter, the solvent was distilled off under reduced pressure to obtain 472 g of second surface-treated silica particles [3].

[0086] [Synthesis Example 4] The same procedure as in Synthesis Example 3 was carried out except that in step (S1), the hydrolysis temperature of tetramethoxysilane was set to 27° C., thereby obtaining 469 g of surface-treated silica particles [4].

[0087] [Synthesis Example 5] The same procedure as in Synthesis Example 3 was carried out except that the hydrolysis temperature of tetramethoxysilane was set to 20° C. in step (S1), thereby obtaining 461 g of surface-treated silica particles [5].

[0088] [Comparative Synthesis Example 1] A 0.3-liter glass reactor equipped with a stirrer and thermometer was charged with 100 g of deflagration silica (product name: SO-C1, manufactured by Admatechs Co., Ltd.), and 1 g of purified water was added with stirring. After sealing, the reactor was further stirred at 60°C for 10 hours. After cooling to 25°C, 2 g of hexamethyldisilazane was added with stirring. After sealing, the reactor was further stirred for 24 hours. The temperature was raised to 120°C, and the remaining raw materials and generated ammonia were removed while passing nitrogen gas through the reactor, yielding 100 g of surface-treated silica particles [6].

[0089] [Comparative Synthesis Example 2] A 0.3-liter glass reactor equipped with a stirrer and thermometer was charged with 100 g of deflagration silica (product name: SO-C1, manufactured by Admatechs Co., Ltd.), 1 g of purified water was added with stirring, and the reactor was sealed and stirred at 60°C for an additional 10 hours. After cooling to 25°C, 1 g of methyltrimethoxysilane was added with stirring, the reactor was sealed and stirred for an additional 24 hours. Next, 2 g of hexamethyldisilazane was added with stirring, the reactor was sealed and stirred for an additional 24 hours. The temperature was raised to 120°C, and the remaining raw materials and generated ammonia were removed while nitrogen gas was passed through, yielding 101 g of surface-treated silica particles [7].

[0090] [Comparative Synthesis Example 3] The same operation as in Synthesis Example 1 was carried out, except that in step (S4), hexamethyldisilazane was not added and the solvent in this dispersion was distilled off at 130°C under reduced pressure (6,650 Pa), thereby obtaining 179 g of surface-treated silica particles [8].

[0091] [Comparative Synthesis Example 4] A 0.3-liter glass reactor equipped with a stirrer and a thermometer was charged with 500 ml of ethylenediaminetetraacetic acid (BET)-based ammonium hydroxide (MAA) produced by a gas-phase method. 2100 g of silica powder (100 g / g) was charged, 1 g of pure water was added with stirring, the container was sealed, and the mixture was further stirred at 60°C for 10 hours. After cooling to 25°C, 2 g of hexamethyldisilazane was added with stirring, the container was sealed, and the mixture was further stirred for 24 hours. The temperature was raised to 120°C, and the remaining raw materials and the generated ammonia were removed by passing nitrogen gas through the mixture, yielding 100 g of surface-treated silica particles [9].

[0092] The surface-treated silica particles (surface-treated silica particles [1] to [9]) obtained in Synthesis Examples 1 to 5 and Comparative Synthesis Examples 1 to 4 were measured according to the following methods. The results are shown in Table 1.

[0093] [Particle size] The surface-treated silica particles were added to methanol to a concentration of 0.5% by mass, and the particles were dispersed by applying ultrasonic waves for 10 minutes. The volumetric particle size distribution was measured using a dynamic light scattering / laser Doppler method Nanotrac particle size distribution analyzer (product name: UPA-EX150, manufactured by Nikkiso Co., Ltd.), and the median diameter (50% cumulative diameter) in the particle size distribution was calculated.

[0094] [Shape observation] The particles were observed using an electron microscope (product name: S-4700 model, magnification: 100,000 times, manufactured by Hitachi, Ltd.) to confirm their shapes. Particles with a circularity of 0.8 to 1 when projected two-dimensionally were classified as "spherical," and any other shape was classified as "irregular." Here, circularity refers to (the perimeter of a perfect circle equal to the area of ​​the particle when projected two-dimensionally) / (the perimeter of the area of ​​the particle when projected two-dimensionally), and the average value was calculated by measuring 100 primary particles.

[0095] [Table 1]

[0096] [2] Manufacturing of coating materials for secondary battery separators [Examples 1-1 to 1-5, Comparative Examples 1-1 to 1-4] The surface-treated silica particles (2 mass%, 5 mass%, 10 mass%, 20 mass%, 30 mass%) obtained in Synthesis Examples 1 to 5 and Comparative Synthesis Examples 1 to 4 above, 5 mass% of a VDF-HFP copolymer (KYNAR (registered trademark) 2851, manufactured by ARKEMA) as a binder, and NMP as a solvent were mixed, and the mixture was degassed in a rotary whisk at 2,000 rpm for 5 minutes to obtain a coating material for a secondary battery separator.

[0097] [3] Manufacturing and evaluation of secondary battery separators [Examples 2-1 to 2-15, Comparative Examples 2-1 to 2-31] 10×10cm 2 The coating material for secondary battery separator obtained above was applied to one side of a nonwoven fabric (a 15 μm thick aramid fiber nonwoven fabric that had been previously heated at 150° C. for 12 hours and dehydrated) cut to size in the amount shown in Tables 2 and 3, and the excess was scraped off with a squeegee. After that, the coating material was dried in an oven at 150° C. for 12 hours to obtain a secondary battery separator. 2 The coating amount of the surface-treated silica particles per separator was calculated from the mass of the separator before and after coating and drying.

[0098] The thickness of the obtained secondary battery separator, the results of the combustion test, and the lithium ion battery properties are shown in Tables 2 and 3. The results of a secondary battery separator in which no coating material was applied to the nonwoven fabric are shown in Comparative Example 2-31.

[0099] [Separator thickness] The thickness of the resulting secondary battery separator was measured with a thickness gauge. A thickness of 24 μm or less was rated as ◯, a thickness of more than 24 μm but not more than 30 μm was rated as △, and a thickness of more than 30 μm was rated as ×.

[0100] [Combustion test] The resulting secondary battery separator was cut to a size of 3.5 x 6.5 cm, and both ends in the longitudinal direction (6.5 cm direction) were fixed with two stands so that the separator was parallel and maintained in a shape without loosening. The separator was ignited from the lower center and left until the fire was extinguished, after which the separator was visually inspected, and those that maintained their shape were marked with a ◯, and those that could not maintain their shape and separated were marked with an ×.

[0101] [Battery characteristic test] The battery characteristics of the lithium ion secondary battery produced by the following procedure were evaluated.

[0102] (cathode material) LiNi as the positive electrode active material 0.8 Co 0.1 Mn 0.1 The O2 was laminated and bonded to a metal plate, and an extraction electrode (tab) was electrically welded to the metal part that did not have the positive electrode active material to form the positive electrode material. Polyimide tape was attached to the entire back side, which was not in close contact with the separator, to provide electrical insulation.

[0103] (Anode material) Graphite was laminated and bonded to a metal plate as the negative electrode active material, and an extraction electrode (tab) was electrically welded to the metal part that was not coated with the negative electrode active material to form the negative electrode material. Polyimide tape was attached to the entire back side, which was not in close contact with the separator, to provide electrical insulation.

[0104] (Lamination of lithium-ion battery components) The polyolefin-coated aluminum film was placed with the polyolefin film facing up, and the negative electrode material, the secondary battery separator obtained above, and the positive electrode material were laminated in that order. The aluminum film in the portion not covered by the laminate was folded to form an exterior seal. The separator was placed so that the surface coated with the coating material faced the positive electrode.

[0105] (Thermal welding of the aluminum film edge) The end faces of the aluminum film were each heated and pressed at 180°C to seal them. The end faces where the extraction electrodes were not exposed were left open and not sealed. The laminate thus obtained was dried under reduced pressure at 130°C for 12 hours.

[0106] (Electrolyte filling) After drying, the laminate was filled with dry N2 in a glove box, and an electrolyte solution was poured into the end surface that was not subjected to pressure bonding. The electrolyte solution used was a 1 mol / L LiPF6 [ethylene carbonate:ethylene carbonate (1:1 vol%)] solution. Thereafter, the open portion was welded using a vacuum heating laminator in a glove box to obtain a lithium ion battery.

[0107] The lithium-ion batteries obtained as described above were subjected to preliminary charge / discharge (chemical conversion treatment), then charged to 4.1 V at a current of 0.2 cA in a 30°C thermostatic chamber, and then charged at a constant voltage of 4.1 V until the current reached 0.02 cA. After charging, the batteries were repeatedly discharged to 2.7 V at a current of 0.2 cA. The battery capacity after 500 charge / discharge cycles was determined, assuming the initial capacity to be 100%, and the retention rate was calculated. A retention rate of 85% or more was evaluated as ◯, a retention rate of 80% or more as △, and a retention rate of less than 80% as ×.

[0108] [Table 2]

[0109] [Table 3]

[0110] As shown in Table 2, when the surface-treated spherical silica particles of Synthesis Examples 1 to 5 were used as a coating material for a secondary battery separator, the coating amount was 0.07 to 0.29 mg / cm 2 Within this range, the separator thickness, combustion test results, and battery characteristics all showed good results (Examples 2-1 to 2-15). On the other hand, the application amount is low at 0.03 mg / cm 2and a high application rate of 0.43 mg / cm 2 In the battery characteristic test, the battery capacity retention rate after 500 charge / discharge cycles was insufficient under the conditions above. 2 The results of the combustion test were unsatisfactory under these conditions. Low application amount: 0.03mg / cm 2 It is thought that the effect of the surface-treated spherical silica particles was insufficient under the conditions of 0.43 mg / cm, causing a short circuit. 2 It is believed that under these conditions, the battery characteristics deteriorated due to an increase in internal resistance.

[0111] As shown in Table 3, when the surface-treated silica particles of Comparative Synthesis Examples 1 to 4 were used as a coating material for secondary battery separators, the total thickness of the separator tended to increase and the battery characteristics were also inferior. These silica particles tend to aggregate, which is thought to be why the film thickness increases when applied to the separator, increasing the internal resistance and causing a decrease in battery characteristics.

[0112] Furthermore, as shown in Comparative Example 2-31, the separator without silica particles showed poor results in the battery characteristic test. This is thought to be due to the occurrence of short circuits. The separator also showed unsatisfactory results in the combustion test.

Claims

1. R on the surface 1 SiO 3 / 2 Units (wherein R 1 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 20 carbon atoms; and R 2 3 SiO 1 / 2 Units (wherein R 2 are the same or different, substituted or unsubstituted monovalent hydrocarbon groups having 1 to 6 carbon atoms, and the coating material for a secondary battery separator contains surface-treated spherical silica particles having a median diameter of 0.01 to 0.5 μm in a volume-based particle size distribution and a circularity of 0.8 to 1.

0.

2. 2. The method for producing a coating material for a secondary battery separator according to claim 1, comprising: Si(OR 3 ) 4 (I) (In the formula, R 3 are the same or different monovalent hydrocarbon groups having 1 to 6 carbon atoms. a tetrafunctional silane compound represented by the formula: 2 After obtaining a mixed solvent dispersion of hydrophilic spherical silica particles containing units, the mixed solvent dispersion of the hydrophilic spherical silica particles is added with a compound represented by the following formula (II): R 1 Si (OR 4 ) 3 (II) (In the formula, R 1 is the same as above, and R 4 are the same or different monovalent hydrocarbon groups having 1 to 6 carbon atoms. The surface of the hydrophilic spherical silica particles is treated with a trifunctional silane compound represented by the formula: 1 SiO 3 / 2 Units (wherein R 1 is the same as above.) is introduced into the first surface-treated spherical silica particles in a mixed solvent, and then the first surface-treated spherical silica particles are concentrated by removing a part of the hydrophilic organic solvent and water from the mixed solvent dispersion to obtain a concentrated mixed solvent dispersion of the first surface-treated spherical silica particles. Then, the first surface-treated spherical silica particles are concentrated in a mixed solvent by adding a compound represented by the following formula (III) to the concentrated mixed solvent dispersion of the first surface-treated spherical silica particles. R 2 3 SiNHSiR 2 3 (III) (In the formula, R 2 is the same as above.) a silazane compound represented by the following formula (IV): R 2 3 SiX (IV) (In the formula, R 2 is the same as above, and X is an OH group or a hydrolyzable group. or a mixture thereof, to treat the surface of the first surface-treated spherical silica particles, thereby forming R 2 3 SiO 1 / 2 Units (wherein R 2 is the same as above.) to obtain surface-treated spherical silica particles as second surface-treated spherical silica particles.

3. A secondary battery separator having a substrate and a coating of the coating material for a secondary battery separator according to claim 1 formed on the surface of the substrate or inside pores of the substrate, 2 The amount of the surface-treated spherical silica particles per separator is 0.07 to 0.29 mg.

4. 4. The secondary battery separator according to claim 3, wherein the substrate is a nonwoven fabric.

5. A secondary battery comprising the secondary battery separator according to claim 3 or 4.

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